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Chapter 17
Lipids and Their Functions in
Biochemical Systems
Denniston
Topping
Caret
6th Edition
17.1 Biological Functions of Lipids
• As an energy source, lipids provide 9 kcal
of energy per gram
• Triglycerides provide energy storage in
adipocytes
• Phosphoglycerides, sphingolipids, and
steroids are structural components of cell
membranes
• Steroid hormones are critical intercellular
messengers
• Lipid-soluble vitamins (A, E, D, K)
• Dietary fat acts as a carrier of lipid-soluble
vitamins into cells of small intestine
• Provide shock absorption and insulation
17.1 Biological Functions
of Lipids
Classification of Lipids
Four Main Groups
• Fatty Acids
– Saturated
– Unsaturated
• Glycerides glycerol-containing lipids
• Nonglyceride lipids
– Sphingolipids
– Steroids
– Waxes
• Complex lipids lipoproteins
17.1 Biological Functions
of Lipids
A Scheme to Classify Lipids
17.2 Fatty Acids
• Long straight-chain carboxylic acids
– no branching
• Most common chains range from 10–20
carbons in length
• Usually, an even number of carbons in
the chain, including the carboxyl carbon
• Can be saturated or unsaturated, but
usually no other functional groups
present
– Any fatty acid that cannot be synthesized
by the body is called an essential fatty acid
17.2 Fatty Acids
Structure
• Stearic acid: a typical
saturated fatty acid with
18 carbons in the chain
• Oleic acid: a typical
unsaturated fatty acid
with 18 carbons in the
chain
17.2 Fatty Acids
Saturated and Unsaturated
Fatty Acids
• Saturated fatty acids have no double bonds
• Unsaturated fatty acids do contain double
bonds
• The double bond is normally in a cis
configuration
• Double bonds lower the melting temperature
– The cis configuration doesn’t allow fatty
acids to pack as close together
17.2 Fatty Acids
Fatty Acids
• An unsaturated fatty acid has one or more
carbon-carbon double bonds in the chain
• The first double bond is usually at the ninth
carbon
• The double bonds are not conjugated and
are usually cis
• cis double bonds result in a bent chain and
O
H
lower melting point H
C C
CH2
CH2
CH2
CH2
CH2
CH3
CH2 CH2 CH2 C
O
CH CH2 CH2 CH2
2
Palmitoleic
acid
17.2 Fatty Acids
Fatty Acid Properties
• Melting point increases with increasing carbon
number
• Melting point of a saturated fatty acid is higher
than an unsaturated fatty acid with the same
number of carbons
• Typical saturated fatty acids are tightly packed
together
• cis double bonds prevent good alignment of
molecules in unsaturated fatty acids leading to
poor packing
• Double bonds lower melting point relative to
saturated acid
17.2 Fatty Acids
Common Fatty Acids
17.2 Fatty Acids
Melting Points of Fatty Acids
17.2 Fatty Acids
Chemical Reactions of
Fatty Acids
Esterification reacts fatty acids with
alcohols to form esters and water
17.2 Fatty Acids
Fatty Acid Hydrolysis
• Acid Hydrolysis reverses esterification
– Fatty acids are produced from esters
17.2 Fatty Acids
Saponification
• Saponification is the base-catalyzed
hydrolysis of an ester
• Products of the reaction are
– An alcohol
– An ionized salt which is a soap
• Soaps have a long uncharged hydrocarbon
tail
• Also have a negatively charged carboxylate
group at end
• Form micelles that dissolve oil and dirt
particles
17.2 Fatty Acids
Saponification Problems
• When “hard” water is used with soaps
– “Hard” water contains high concentrations of
Ca2+ and Mg2+
• Cations in the water form fatty acid salts
which precipitate
– Interferes with emulsifying action of the soap
– Leaves a crusty scum on the surface of the
sink
17.2 Fatty Acids
Reaction at the Double Bond
• Hydrogenation is an addition reaction
• Unsaturated fatty acids can be converted
to saturated fatty acids
• Hydrogenation is used in the food industry
O
CH3 CH2 4CH CH CH2CH CH CH2 7 C OH
2 H2, Ni
O
CH3 CH2 16 C OH
Eicosanoids: Prostaglandins,
17.2 Fatty Acids
Leukotrienes, and Thromboxanes
• Fatty acids which can’t be synthesized by the
body are essential fatty acids
– Linoleic acid is an essential fatty acid required to
make arachadonic acid
• Arachidonic acid (20 C) is the eicosanoid
precursor
• Eicosanoids are three groups of structurally
related compounds
COO
– Prostaglandins
– Leukotrienes
– Thromboxanes
arachadonic acid
17.2 Fatty Acids
Prostaglandins
• Potent biological molecules
• They act like hormones in controlling the
body’s processes
• Structure
– Synthesized from 20-carbon unsaturated fatty
acids
– Cyclic compounds including a 5-carbon ring
• Names are based on ring substituents and
number of side-chain double bonds
• Made in most tissues
– Exert their effects on cells that produce them and
cells in the immediate vicinity
17.2 Fatty Acids
Biological Processes Regulated
by Eicosanoids
1. Blood clotting
– Thromboxane A2 stimulates constriction
of blood vessels and platelet aggregation
– Prostacyclin dilates blood vessels and
inhibits platelet aggregation
2. Inflammatory response
– Prostaglandins mediate aspects of
inflammatory response
3. Reproductive system
– Stimulation of smooth muscle by PGE2
17.2 Fatty Acids
Biological Processes
Regulated by Eicosanoids
4. Gastrointestinal tract
–
–
–
Prostaglandins inhibit gastric secretion
Prostaglandins increase secretion of protective
mucus
Inhibition of hormone-sensitive lipases
5. Kidneys
–
–
Prostaglandins dilate renal blood vessels
Results in increased water and electrolyte
excretion
6. Respiratory tract
–
–
Leukotrienes promote the constriction of bronchi
Prostaglandins promote bronchodilation
17.2 Fatty Acids
Structures of Four Prostaglandins
17.2 Fatty Acids
Thromboxane and Leukotriene
Structure
17.2 Fatty Acids
Aspirin and Prostaglandins
Aspirin inhibits prostaglandin synthesis by
acetylating cyclooxygenase, an enzyme
necessary for prostaglandin synthesis
17.2 Fatty Acids
Overview of Prostaglandin
Synthesis From Arachidonic Acid
17.3 Glycerides
• Glycerides are lipid esters
– Alcohol group of glycerol form an ester with a
fatty acid
– Esterification may occur at one, two, or all
three alcohol positions producing:
• Monoglyceride
• Diglyceride
• Triglyceride
– A neutral triacylglycerol or a triglyceride
– Triglycerides are nonionic and nonpolar
– Triglycerides serve as energy storage in
adipose cells
17.3 Glycerides
Triglycerides
• Glycerides are lipid esters
• A triglyceride places fatty acid chains at
each alcohol group of the glycerol
O
CH2O C R1
O
Glycerol
part CH O C R2
O
CH2O C R3
Fatty acid
chains
17.3 Glycerides
Chemical Properties
Triglycerides have typical ester and
alkene chemical properties as they are
composed of these two groups:
– Saponification: replace H with salt from a
strong base
– Hydrolysis: produces the fatty acids and
glycerol, a reverse of formation
– Hydrogenation: saturates the double
bonds
17.3 Glycerides
Triglyceride Reactions
• Triglycerides undergo three basic reactions
• These reactions are identical to those
studied in carboxylic acids
Triglyceride
H2, Ni
H2O, H+
NaOH
Glycerol
Fatty Acids
Glycerol
Fatty Acid Salts
More saturated
triglyceride
17.3 Glycerides
Fats and Oils
• Triglycerides or triacylglycerols
– Fats are a combination of glycerol and the
fatty acids
• Fats mainly come from animals,
unless from fish, and are solid at room
temperature
• Oils mainly come from plants, and are
liquid at room temperature
17.3 Glycerides
Phosphoglycerides
• Phospholipid is a more
general term
– Any lipid containing
phosphorus
• Phosphoglycerides contain:
– Glycerol
– Fatty acid
– Phosphoric acid with an
amino alcohol
• Replace an end fatty acid of
a triglyceride with a
phosphoric acid linked to an
amino alcohol
G
l
y
c
e
r
o
l
Fatty Acid
Fatty Acid
Phosphoric Acid
Alcohol
17.3 Glycerides
Phosphoglycerides
• Have hydrophobic and hydrophilic
domains
• Structural components of membranes
• Emulsifying agents
• Suspended in water, they
spontaneously rearrange into ordered
structures
– Hydrophobic group to center
– Hydrophilic group to water
– Basis of membrane structure
17.3 Glycerides
Types of Phosphoglycerides
• The phospho-amino-alcohol is highly
hydrophilic
• They are used in:
– Cell membranes
– Emulsifying
– Micelle-forming agents in the blood
• Two types
– Ones made with choline are called lecithin
– Those made with either ethanolamine or serine
are called cephalins
17.3 Glycerides
Types of Phosphoglycerides
17.4 Nonglyceride Lipids
Sphingolipids
• These lipids are based on sphingosine
– Long-chain
– Nitrogen-containing
– Alcohol
• Amphipathic, like phospholipids
– Polar head group
– Two nonpolar fatty acid tail
• Structural component of cellular membranes
• Major categories
– Sphingomyelins
– Glycosphingolipids
17.4 Nonglyceride Lipids
Types of Sphingolipids
• Sphingomyelins
– Structural lipid of nerve
cell membranes
– Myelin sheath feature
• Glycosphingolipids
– Built on a ceramide
– Cerebrosides have a
single monosaccharide
head group
• Glucocerebroside
• Galactocerebroside
17.4 Nonglyceride Lipids
Sphingolipid Storage
Diseases
Disease
Symptom
Sph. Lip
Enzyme
Tay-Sachs Blindness,
Ganglioside
muscles weak GM2
b-hexoseaminidase A
Gaucher’s
Liver & spleen Glucoenlarge, MR
cerebroside
b-glucosidase
Krabbe’s
demyelation,
MR
Galactocerebroside
bgalactosidase
NiemanPick
MR
Sphingomyeli Sphingomyelina
se
n
17.4 Nonglyceride Lipids
Steroids
• Steroids are synthesized from the five
carbon isoprene unit (see Alkenes)
• Part of a diverse collection of lipids
called isoprenoids / terpenes
• Contain the steroid carbon skeleton
– A collection of 5 fused carbon rings
17.4 Nonglyceride Lipids
Steroids
• Cholesterol
– Cell membranes, precursor to bile
salts, male and female sex hormones,
vitamin D, and the adrenocortical
hormones
– Have been linked to atherosclerosis
• Bile salts
– Important in the lipid digestion
17.4 Nonglyceride Lipids
Steroid Examples
OH
CH3
CH3
CH3
Cholesterol
CH3
CH CH2 CH2 CH2 CH(CH3)2
H
CH3
H
H
H
O
HO
testosterone
H
CH3
C O
progesterone
CH3
O
CH3
17.4 Nonglyceride Lipids
Steroid Hormones
• Adrenocorticoids have two groups
– Mineralocorticoids regulate ion
concentrations
– Glucocortiods enhance carbohydrate
metabolism
• Cortisol increases glucose and glycogen
in the body
– Along with its ketone derivative, cortisone;
are anti-inflammatory
– Another derivative is prednisolone for both
asthma and inflammation
17.2 Fatty Acids
Waxes
• Waxes are also esters like fats
• Use one alcohol instead of glycerol
• Esters of:
– Long-chain fatty acid
– Long-chain alcohol
• The longer the chains, the higher the
melting point
• Protects the skin of plants and fur of
animals
• Examples of waxes include:
– Carnuba, from Brasilian wax palm
– Beeswax
17.5 Complex Lipids
Complex lipids are those bonded to other
types of molecules
•Lipoproteins
– Molecular complexes found in blood plasma
– Contain:
• Neutral lipid core of cholesterol esters and/or
TAGs
• Surrounded by a layer of:
– Phospholopid
– Cholesterol
– Protein
•Classes: chylomicrons, VLDL, LDL, HDL
17.5 Complex Lipids
Major Classes of Lipoproteins
• Chylomicrons:
– Very large and very low density
– Transport intestineadipose
• VLDL:
– Made in liver
– Transport lipids to tissues
• LDL: carry cholesterol to tissues
• HDL:
– Made in liver
– Scavenge excess cholesterol esters
– “Good cholesterol”
17.5 Complex Lipids
Model Structure of a
Plasma Lipoprotein
17.5 Complex Lipids
Relative Composition of
Lipoproteins
17.5 Complex Lipids
Membrane Receptors
• The LDL receptor was discovered during an
investigation of familial hypercholesterolemia
• When a cell needs cholesterol, it synthesizes
the receptor, which migrates to a coated
region of the membrane
• The “captured” cholesterol is absorbed by
endocytosis
• Failure to make the receptor is the most
common problem encountered
17.5 Complex Lipids
Receptor-mediated Endocytosis
17.6 The Structure of Biological
Membranes
• Each type of cell has a unique membrane
composition with varying percentages of
lipids, proteins, and some carbohydrates
• The currently accepted model of the
membrane is the fluid mosaic model of a
lipid bilayer
17.6 The Structure of
Biological Membranes
17.6 The Structure of
Biological Membranes
Membrane Proteins
• Most membranes require proteins to
carry out their functions
• Integral proteins are embedded in
and/or extend through the membrane
• Peripheral proteins are bound to
membranes primarily through
interactions with integral proteins
17.6 The Structure of
Biological Membranes
Fluid Mosaic Model of
Membrane Structure